Polyhaloimides, especially tetrabromophthalimides and bis(tetrabromophthalimides), are well-known compounds which find an application as flame retardants in many inflammable substances, especially in plastics (see, for example, the articles by S. M. SPATZ et al. entitled "Some N-substituted tetrabromophthalimide fire-retardant additives" in Industrial and Engineering Chemistry Product Research and Development, vol. 8, No. 4 (1969) pages 397-398, as well as U.S. Pat. No. 3,873,567, French patent 2,369,261 and Japanese application Nos. 74-045,062 and 75-064,337.
However, the preparation processes described in the references mentioned above give poor yields and provide products which are most frequently yellow in color or which become colored when they are used and thereby confer to plastics (compounds or molded articles) a color which is redhibitory and unacceptable in a good many uses. Moreover, these products very often contain substances which are volatile at the temperatures at which they are used in some polymeric substances and lead to the corrosion of molds. Additionally, the use in their preparation of organic solvents (especially xylene, toluene, alcohol, acetic acid), most frequently chosen because of their capacity to form azeotropic mixtures with water which enable the water of condensation produced by the imidification reaction to be carried over or to dissolve the halogenated dicarboxylic acid anhydride, requires expensive operations for the separation and the recovery of these solvents, as well as drying means suitable for the removal of the organic solvent vapors.
The disadvantages mentioned above are encountered especially in the case of polyhaloimides derived from hydrazine and halogenated dicarboxylic acid anhydrides. It has now been found that it is not indispensable to use an organic solvent to dissolve the anhydride and/or remove the water of condensation and that by operating in an aqueous medium under certain conditions, colorless or very slightly colored products which, without prior purification, are perfectly suitable for flameproofing macromolecular substances, including those which are employed at high temperature, especially above 250.degree. C., may be obtained with very high yields and without effluent disposal and environmental problems.